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Maiduguri is plastered with the faces of alleged high-level Boko Haram fighters, collages of photos on banners slung across walls and fences and storefronts. Some of the militants are carrying rifles. Others are wearing military fatigues.

"Wanted Boko Haram Suspects by Nigerian Army," the banners read.

The pictures of the alleged militants are small. You have to get close to register one of the banners' most jarring features: A number of those top-tier suspects are actually boys.

Some look no older than 14. They smile at the camera or wave scrawny arms at the photographer. They wear ski caps too big for their heads.

How did a bunch of teenagers become the most wanted people in Africa's most populous country?

Beginning in 2011, Boko Haram, an Islamist extremist group, swept through an enormous stretch of Nigeria and into neighboring Cameroon and Chad, growing larger and more destructive.

This was a group that delivered nothing to its new subjects other than rape and murder. Yet each year its campaign expanded wider and wider, mystifying foreign officials and aid workers, who asked: Who are these men?

It was that question that the Nigerian Army sought to answer when it designed the wanted posters. In the centre it put Abubakar Shekau, the erratic, bearded leader who screeched threats in Arabic on the group's videos. But beyond Shekau, officers believed, some of Boko Haram's most dangerous members were barely adolescents. The soldiers didn't hesitate to add the boys' photos to the banner - they were wanted, too.

"In banditry, age is not an issue," said Major General Lucky Irabor, the top military official in northeastern Nigeria, defending the use of the boys' photos.

Today is the second anniversary of one of Boko Haram's most notorious crimes, the abduction of 276 schoolgirls in the town of Chibok. The kidnapping generated international outrage, with millions of people - including US first lady Michelle Obama - tweeting the phrase #bringbackourgirls. But the schoolgirls, most of whom are still missing, are hardly the only casualties of Boko Haram.

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Since 2014, a fifth of all Boko Haram suicide bombers have been children, according to a report from Unicef released this week. And Nigerian officials say that's not the only threat children pose.

"They can lead. They can fight. They can cause all kinds of damage," said a senior Nigerian military officer. Boko Haram has suffered major defeats over the past year and has lost much of the territory it controlled. But the group is still a threat, carrying out frequent terror attacks.

Still, putting a boy's face on a wanted poster seems to some an inappropriate move, particularly as the military has made clear that top Boko Haram suspects are wanted dead or alive. And according to experts, many of the boys have been brainwashed or otherwise coerced into fighting.

"When the photos of these boys are displayed, it perpetuates the fear around these children. Because they are under 18, we consider them victims," said Rachel Harvey, Unicef's head of child protection in Nigeria.

A Boko Haram's 'Most Wanted' poster hangs on a gate in Maiduguri, Nigeria. Photo/ The Washington Post by Jane Hahn.

A Boko Haram's 'Most Wanted' poster hangs on a gate in Maiduguri, Nigeria. Photo/ The Washington Post by Jane Hahn.

In its report, Unicef claimed that many of the boys who are considered Boko Haram fighters are actually acting under enormous pressure.

"Boys are forced to attack their own families to demonstrate their loyalty to Boko Haram," the report said. For the children, the alternative is often death.

Other researchers have found that some boys initially joined Boko Haram because the group claimed to espouse Islamic values in a mixed Christian-Muslim country where Muslims have often complained of discrimination. In some cases, Boko Haram militants offered boys cash loans for joining.

But as the movement's ultra-violent modus operandi became clearer, the reasons for enlistment changed. For many boys, not joining meant that their homes and family businesses would be destroyed - and that they would quickly become targets.

"Because I needed an identity to remain safe, I decided to pledge my allegiance to them," one young former member told a researcher for the international aid group Mercy Corps. "At that time I needed protection and immunity from persecution by them so I could continue with my business."

Sub-Saharan Africa offers plenty of examples of the boy-commander. During Liberia's civil wars in the 1990s and 2000s, pre-pubescent boys were put in charge of units of even younger children and often were sent directly to the front lines.

"As a commander, I was in charge of nine others, four girls and five boys. We were used mostly for guarding checkpoints but also fighting. I shot my gun many times," a Liberian boy named Patrick told Human Rights Watch in 2004. He was 12 when he fought in the war.

In Mozambique, during the conflict that raged there from 1977 to 1992, more than 25 percent of fighters were believed to be younger than 18. And in South Sudan today, hundreds of children fight on behalf of the government and the rebels.

With their wanted posters, Nigerian officials seem to have made it clear that many of the boys aren't just foot soldiers. They are more serious threats. And authorities do not appear swayed by the idea that the young men may be victims themselves.